This invention relates to an electrophoretic display device. The principle of operation of such a display is based on two phenomena. They are:
(1) Charged particles, when suspended in a liquid medium and subjected to a uniform electric field, experience a force and undergo translation along the field.
(a) When such particles arrive at the wall of the cell they will adhere to the wall due to some combination of forces which may include van der Walls attraction and electrostatic forces.
If the particles are micron-sized and have a density comparable to the density of the liquid medium, they will remain in suspension for extended periods of time without settling out. In a display application, the liquid may be dyed some dark color, e.g. black. On the other hand, a color contrasting to the liquid will be chosen for the particles. Typically, Titanium dioxide has been used for its high reflectance white color. The cell may consist of two glass plates separated by a spacing of around 0.001" and sealed around the edges. Electrodes on the inside of the glass plates may be formed by means of transparent Tin oxide layers. In the unenergized state, the cell appears black, i.e., the color of the liquid. When a potential is applied to the opposing electrodes, the particles migrate to one wall and cause the appearance of that wall to change to white. If the potential is now removed, the particles continue to stay on the wall indefinitely. A reversal of the polarity of applied voltage causes the particles to leave the wall and travel to the opposite wall. If the electrode on one wall is in the shape of a character, then that character can be made to appear alternately with the above sequence of potentials applied.
Simple displays of such configuration have been fabricated by many for laboratory demonstrations of several numeric digits per display. In each case every segment of every digit was driven by a separate driver circuit.
The structure and principle of operation of a usual electrophoretic display panel is described in detail, for example in U.S. Pat. No. 3,668,106. The electrophoretic display panel comprises a cell formed by two opposed transparent insulating substrates which have transparent electrodes formed thereon, respectively and an electrophoretic suspension, which consists of fine particles of colored electrophoretic material suspended in a colored suspending medium, in said cell. When a D.C. voltage is applied to the cell, the particles are moved and deposited on one electrode according to the polarity of the electrophoretic material, and the thus formed image is observed by reflective light.
In a conventional electrophoretic display panel, the thickness of the suspension layer can be anywhere from a few microns to several mils and the display is usually operated by a D.C. voltage. The electro-optical transfer characteristic of such a display normally does not make possible an X-Y matrix selection of cells. This is caused by a characteristic of the electrophoretic cell whereby the change of optical state is time dependent but not voltage dependent. That is, nearly any small voltage across a cell will cause it to eventually change its optical state. The magnitude of the applied voltage merely affects the time it takes for the optical change of state to occur. Furthermore, the effect of repeated application of a small potential is cumulative and with sufficient cumulative volt-seconds applied a given cell will change its optical state.
One fundamental requirement of a display cell when connected in a matrix addressing fashion is to exhibit a definite threshold, that is a highly non-linear electro-optic transfer characteristic. When a voltage below the threshold value is applied to the cell repeatedly, the optical state of the cell must change. However, when twice the previous value of potential is applied, the cell is required to quickly change its optical state. If this condition is not met, then the display will show severe crosstalk. That is, elements which are not required to produce a picture will begin to turn on and elements which are a required part of the picture may begin to turn off.
In a conventional multiplexed matrix addressed electroluminescent display, this problem is overcome by an inherently highly non-linear electro-optic transfer characteristic exhibited by thin film electroluminescent cells. If the normal excitation of such cell is a voltage of value E which produces a light output of value Q then 1/2 E will produce a light output of 10.sup.-3 .times.Q. This highly non-linear transfer characteristic of the electroluminescent cell makes its use in matrix displays possible.
It is possible to connect in series with a display cell another device that has a highly non-linear volt-ampere characteristic and thereby enhance the non-linear electro-optic transfer characteristic of the series combination. Typical of such non-linear devices are Diodes, Zener Diodes, Varistors and the like. A disadvantage of such combinations is the increased complexity of the overall display, increased manufacturing difficulties and increased cost.
It is therefore much more desirable to find a display cell which inherently exhibits such a threshold or non-linear electro-optic transfer characteristic.
Thus, a fundamental limitation of earlier electrophoretic displays was the lack of a threshold in the transfer characteristic. This means that the particles begin to move at the smallest voltage applied. As the voltage is increased the particles move faster. A desired threshold characteristic would be a lack of any translation of the particles at applied voltages below a certain threshold no matter how long or how often that potential may be applied. When the applied potential is nearly twice the threshold potential then the particles should move at useful velocity.
Large matrix displays can be practical only by using X-Y addressing techniques to substantially reduce the total number of driver circuits required. However, X-Y addressing, of course, requires that each display cell exhibit a definite threshold.